The cleaning of machined metal surfaces by spraying is generally carried out with the use of aqueous cleaners based on organic rustproofing agents, for example, using combinations of alkanolamines and fatty acids, surfactants, antifoam agents and/or phosphonates, optionally in conjunction with biocides and builders. Such cleaners are generally used in the form of aqueous liquids having a pH of from 7.5 to 10.5. Because of this, they are generally called "neutral cleaners".
To avoid undersirable corrosion phenomena, it is often advisable to passivate the metal surface with aqueous solutions containing appropriate chemical additives. This is best done during or after cleaning operations, during machining, or before intermediate storage pending further treatment of the metals.
In the absence of further additives, conventional neutral cleaners and corrosion inhibitors are only suitable for the treatment of iron and steel. Cleaning and passivating agents such as these, which inhibit the corrosion of iron and steel surfaces, contain for example alkali nitrites, alkanolamines, soaps, and benzoates. In addition to ferrous metals, non-ferrous metals and lightweight metals and their alloys, particularly zinc, aluminium and its alloys, are increasingly coming into contact with cleaning and passivating agents. Inhibitors available for non-ferrous metals include, for example, mercaptobenzthiazole and benzotriazole. Unfortunately, these passivating components are attended by the disadvantage that they do not prevent discoloration (i.e., springwater blackening) of the surfaces of lightweight metals. In addition, use of these cleaners on zinc, aluminium and its alloys, results in serious metal erosion which cannot be tolerated in the case of precision parts. In the automotive field, this problem is especially troublesome with respect to the new types of engines and transmissions of lightweight metal alloys.
On the other hand, the trend toward atuomation has produced a demand for cleaner systems not suffering from the above disadvantages.
To enable the cleaning liquids to be automatically controlled and metered, even in tapwater, which is generally done by conductivity measurements, the cleaners hitherto used contain polyphosphates which degrade into pyrophosphates and orthophosphates, especially at elevated temperatures. In many cases, the use of inorganic ions such as these results in precipitates with the salts responsible for the hardness of water, i.e. in deposits of calcium or magnesium phosphate, and--in the presence of aluminum alloys--in the precipitation of aluminium phosphate. This in turn results not only in coatings on the parts to be machined, but also in the adhesion, encrustation or even blockage of machinery components and pumps. In addition, precipitating phosphates, particularly aluminium phosphates, can initiate undesirable coagulation processes, including for example the irreversible binding of surfactants or antifoam agents. This in turn leads to a distinctly greater tendency towards foaming, so that it is not long before the cleaner solution is unfit for further use. Also, as pointed out above, these electrolyte-containing products generally cause increased metal erosion and also very rapid discoloration of the lightweight or nonferrous metal.